1. Field of Invention
The invention relates to a video signal capturing apparatus used in a system in which video signals captured from, for example, a digital camera or the like, are image-processed or printed.
2. Description of Related Art
Many systems have been developed in which video signals (in this case, static picture image signals in, for example, the NTSC system) captured from a camera or the like, are image-processed in a predetermined manner by a device such as a personal computer or printed out by a printing mechanism. The camera may be a digital camera including a digital video signal output and an analog signal output which both output the same information in digital and analog format respectively. The NTSC system is an example of analog format.
Video signals systems such as a NTSC system use a four-field sequence as described below. An NTSC system employs an interlace scanning method in which a frame is formed by two vertical sweeps and 525 horizontal sweeps. Interleaving is performed to reduce the influence of chroma components on luminance components. That is, a frequency of the color subcarrier waves (i.e., the subcarrier wave for the chrominance signals) is selected such that the multiple product of an odd number and the half period of the subcarrier waves equals the horizontal sweep time duration. Therefore, the color subcarrier waves are completely out of the phase, i.e., inverted 180xc2x0, for every horizontal scanning line. Since the total number of horizontal lines scanned for a single frame by two fields, is 525, which is an odd number, the phase of the color subcarrier waves is opposite for every frame. More specifically, the polarity of the sine waveform at a given horizontal position on each scanning line alternates for every frame. Since the total number of horizontal lines scanned for two frames by four fields is 1050, which is an even number, the color subcarrier waves and the horizontal lines are in phase every two frames. Thus, taking multiplex chrominance signals into account, the video signals of the NTSC system are in four-field sequences in which a single sequence, including the phase of the color subcarrier waves (frequency fsc=3.58 MHz), is completed by four fields even in the case of a static picture image. The four fields are termed color fields 1 to 4, respectively.
Conventional apparatuses for processing NTSC video signals employ a circuit for distinguishing an odd-numbered field from an even-numbered field. However, these apparatuses are not able to distinguish odd-numbered or even-numbered fields which are 180xc2x0 out of phase from the different color subcarrier wave. For example, those apparatuses cannot distinguish color fields 1 and 3 from each other. Odd-numbered or even-numbered fields, such as color fields 1 and 3, may be distinguished from each other, by employing a phase detection mechanism. However, the phase detection mechanism inconveniently complicates the circuit construction of the apparatus.
Mere determination of even-numbered and odd-numbered fields while not differentiating between the color fields of odd numbers or even numbers, for example, between color field 1 and color field 3, does not present any significant or practical problems-in applications such as television. However, this method for distinguishing fields causes a significant problem in the pursuit of higher image quality.
For example, differentiation between even and odd numbered color fields is important in the pursuit of higher image quality in which a three-dimensional YC separation method or the like is employed. Whereas, for a two-dimensional YC filter, the range of extraction of peripheral components of the color subcarrier waves as chrominance signals is limited to the dimensions of horizontal frequency and vertical frequency, a three-dimensional YC separation filter must be provided with a time-frequency characteristic to take into account the relationship between a field in one frame and the corresponding field in another frame. However, in four-field sequence systems, the color subcarrier waves of two odd-numbered fields, or two even-numbered fields of two consecutive frames are completely opposite in phase to each other. Therefore, a three-dimensional YC separation filter cannot be directly employed in the four-field sequence system. That is, color fields 1 and 3 and color fields 2 and 4 need to be distinguished and separated prior to processing by the three-dimensional YC separation filter.
While the problems of the conventional art have been described with reference to the NTSC video signals, substantially the same problems occur with the video signals of other systems, such as a phase alternation line (PAL) system.
Accordingly, it is an aspect of the invention to provide a video signal capturing apparatus capable of distinguishing fields in a color field unit with a simple construction.
According to the invention, there is provided a video signal capturing apparatus including a converter unit that converts an input video signal into digital video data and a video data writing control unit that performs control such that the digital video data converted by the converter unit is written in a memory. This video signal capturing apparatus captures video data for an external processing apparatus that uses the digital video data stored in the memory. A separator unit separates a vertical synchronizing signal from the input video signal. A determining unit distinguishes a plurality of color fields in the video signal on the basis of a number of vertical synchronizing signals separated by the separator unit. When the determining unit determines that a field corresponds to a field indicated by a setting, a writing instructing unit instructs the video data writing control unit to write the digital video data in the memory.
In the video signal capturing apparatus of the invention, the converter unit converts the input video signal into digital video data, and the video data writing control unit performs control such that the digital video data converted by the converter unit is written in the memory. By writing digital video data in the memory in this manner using the video data writing control unit, video data for an external processing apparatus, such as a personal computer, that uses the digital video data stored in the memory, is captured.
In addition to the basic operation described above, the separator unit separates a vertical synchronizing signal from the input video signal. The determining unit distinguishes a plurality of color fields in the video signal based on a number of vertical synchronizing signals separated by the separator unit. When a field determined by the determining unit corresponds to a field indicated by a setting, the writing instructing unit instructs the video data writing control unit to write the digital video data in the memory.
The video signals in the NTSC system are in four-field sequences in which a single sequence, including the phase of the color subcarrier waves, is completed by four fields even for a static picture image, taking multiplex chrominance signals into consideration, as described above. In the PAL system, video signals are in eight-field sequences. In the case of the NTSC video signals, the four fields are termed color fields 1 to 4. The video signal capturing apparatus of the invention is able to write in the memory only the field that corresponds to the field indicated by the setting. Therefore, the apparatus of the invention can perform video signal capture in which, for example, capture of color field 1 is achieved by the capture of color field 3 or capture of color field 2 is achieved by the capture of color field 4. If color field 1 is captured, color field 3 can be captured merely by determining a color field that occurs two fields after color field 1. If color field 2 is captured, color field 4 can also be captured in the manner as described above.
This operation of the apparatus makes it possible to capture, for example, color field 1, store it in the memory and then transfer it to the personal computer and, after that, capture color field 3, store it in the memory and transfer it to the personal computer. Therefore, the personal computer can obtain color field 3 subsequently to color field 1. Likewise, the personal computer can also obtain color field 4 subsequent to color field 2.
Since the apparatus of the invention is able to capture the four fields separately, the video signal capturing apparatus can capture each color field while shifting the capture timing, i.e., delaying the timing, as needed at the processing side, e.g., in the personal computer, as far as static images are concerned. Since the apparatus is able to perform a capture operation in which, for example, color field 1 is first captured from the input NTSC signals and, after the personal computer completes predetermined processing of the transferred color field, color field 3 is captured, it becomes possible to reduce the required capacity of the memory.
In the invention, the determination or identification of a color field is performed based on the number of vertical synchronizing signals. Therefore, the color field determination may be performed by, for example, counting the number of vertical synchronizing signals. Since each field is led by a vertical synchronizing signal, counting the-number of vertical synchronizing signals enables determination of the relative sequential placement of a given field in, for example, four consecutive fields in the case of the NTSC video signals. Although distinguishing color fields can also be accomplished by detecting the phase of the color subcarrier waves, this type of detection mechanism requires a rather complicated circuit construction. In contrast, the invention requires a much less complicated construction.
The determining unit in the invention merely needs to distinguish a plurality of color fields based on the number of vertical synchronizing signals. The determining unit may include, for example, a scale-of-N counter that counts the number of vertical synchronizing signals separated by the separator unit, wherein N is the number of color fields. For example, a scale-of-four counter is used for the NTSC system and a scale-of-eight counter is used for the PAL system. If the number of vertical synchronizing signals is counted by such a counter, a given count value always indicates the same field among the plurality of fields. Therefore, by capturing a field indicated by, for example, a count value 1 and, after that, capturing a field indicated by a count value 3, field capture can be performed such that the personal computer can obtain, for example, color field 3 subsequent to obtaining color field 1 or color field 4 subsequent to obtaining color field 2.
Although the writing of digital video data in the memory may be performed by a CPU, it is also possible to use a direct memory access (DMS) as a controller for the writing. Since a CPU needs to have a considerably high-speed operating performance in order to perform the writing of the digital video data in the memory, employment of the DMA is normally more practical.
The video signal capturing apparatus of the invention may further include an even-odd number determining unit that differentiates an even-numbered field and an odd-numbered field in interlace scanning of the video signal on the basis of the vertical synchronizing signal separated by the separator unit. In this construction, the writing instructing unit is capable of instructing the video data writing control unit to write the digital video data in the memory in such a manner that the odd-numbered field and the even-numbered field are differentiated and the color fields are distinguished.
If the apparatus of the invention containing structure for determining whether a field is an even-numbered field or an odd-numbered field, the even-numbered fields and the odd-numbered fields can be distinguished so that a plurality of fields can be separately captured based on absolute identification instead of relative identification. In the case of the NTSC video signals, it is determined with regard to each field, whether the field is an odd-numbered field or an even-numbered field. If a field is determined as an odd-numbered field, the odd-numbered field, e.g., color fields 1 or 3, can be identified. Likewise, an even-numbered field, e.g., color fields 2 or 4, can also be identified. With this construction, four fields can be identified and captured and can be processed by the external processing apparatus, while only one memory is used.